DESCRIPTION OF THE PRIOR ART
Polyurethane compositions have been proposed for and used for coating hardwood floors for many years. In the flooring as earlier used, a coating of moisture curing polyurethane (about 40% solids) in a solvent such as xylene or other solvent, was applied to the substrate to saturate it and provide adhesion. After the first coat had at least partially cured, a second coat was applied, and while the second coat was still wet, chips of colored dry paint were scattered over and pressed through the surface. When this layer had dried, the chips which had not adhered were swept away, the surface sanded and vacuumed, and another coat of clear urethane glaze was applied thereover. While that coat was still wet, chips were again scattered over the surface, pressed therein, and when the coating had hardened, the floor was again swept and vacuumed, and a further clear coat of urethane glaze (40% solids) applied. As soon as that coating had hardened, many more coats of 40% solids urethane glaze were applied thereover, each one after the former had hardened.
The flooring as thus produced had these drawbacks: The first coat did not adhere satisfactorily. It was a 40% urethane glaze in hydrocarbon solvent and thus dissolved hydrocarbon soluble stains which were invariably present on the floor, and these stains migrated through to the top of the flooring. Furthermore, the urethane composition invariably turned brown after prolonged exposure to ultraviolet light. Finally the many coats required a great deal of labor, could not be applied in a single day, and the large quantities of solvent such as xylene caused considerable toxicity and odor.
Consequently, the industry evolved the following system:
1. A sealer coat was applied to keep out stains, give adhesion to any substrate, and bond to the next coat. This was either a 100%-solids epoxy or an epoxy emulsion.
2. A chip coat was developed to hold the chips, bridge cracks in the floor, and bond to the next coat. Ordinarily this was the same as the sealer coat.
3. A chip-binding coat or intermediate coat was applied to bind the very hydrophilic chips to each other and to harden them up enough so that they could be sanded. In most cases one or more coats of a polyurethane glaze was used for this purpose, but in some instances a polymeric latex or a clear epoxy emulsion was used.
4. Finally, glaze coats of curing polyurethane in solvent were applied to provide the wearing surface and to give abrasion resistance, stain resistance, and leveling. For each of the coats, an obvious requirement is rapid cure. Without it the job would take too long to be practical.
While the multi-layer system thus proposed is much superior to the original system, it still has many problems. When the epoxy emulsion or 100% solids epoxy is used for both sealer and chip coats, it becomes brittle, shrinks and cracks. This is permissible in a sealer coat, but not a chip coat, which must bridge cracks. In addition, the cure rate of the epoxies is very temperature dependent. Being two package materials, a material with a reasonably long pot life has an inordinately long cure time on a cold floor. Another problem peculiar to the epoxies, when used in urethane systems, is "purpling". The cause is not well understood, but in a significant number of cases the interface between the epoxy and an unpigmented urethane develops an unsightly purple color.
While the currently available base and chip coats present problems, they are more satisfactory than the currently available glazes which have the following faults:
A. The very high xylene content is unsatisfactory for two reasons--the large amount of xylene is unpleasant and dangerous, and the solids content of the glaze is so low that several coats must be applied leading to high labor costs.
B. The glaze yellows badly because of the aromatic isocyanates used. Ultraviolet absorbers effectively halt yellowing only temporarily--a few months to a year--before the film yellows as much as if the absorber were not there.
C. The film formed from the glaze abrades rather quickly.
The obvious solution to the high xylene content, to use less, did not work: Bubbles formed, which eventually broke and collected dirt. Apparently, reducing the xylene content permitted the surface skin to form. This stopped outward diffusion of carbon dioxide which, being entrapped, formed bubbles.
A solution to the yellowing problem is the use of the non-yellowing aliphatic isocyanates used to make polyurethanes. All of these have certain disadvantages. For example, hexamethylene diisocyanate is extremely expensive, is highly toxic, and rather slow to cure. Hydrogenated MDI (HMDI) has two isocyanate groups with equal reactivity. Consequently, it forms highly viscous prepolymers which have a high percentage of free HMDI, which is extremely allergenic via skin absorption.
Isophorone diisocyanate, IPDI, seems to be the best isocyanate available. It is the lowest-priced aliphatic diisocyanate. Its two isocyanate groups are of unequal reactivity so that it gives lower-viscosity prepolymers containing less free monomer vis-a-vis HMDI. However, IPDI has serious drawbacks: although less toxic than HMDI or hexamethylene diisocyanate, it can still cause serious harm via skin absorption. Clear films from IPDI prepolymers degrade to liquid in strong sunlight. Prepolymers formed from the more reactive aliphatic isocyanate group of IPDI are terminated by the less-reactive cycloaliphatic isocyanate groups which moisture cures slowly.
The abrasion resistance problem is probably the most serious. Abrasion resistance is the property which is sought by the purchaser of a floor. None of the solutions mentioned above helped to improve this property. The aliphatic isocyanates, rather than helping improve abrasion resistance, made it worse.
Floor surfaces, particularly those in public buildings, require not only abrasion resistance, but resistance to contamination or staining caused by tar or asphalt brought in by foot traffic from road or parking lot surface. To be a successful floor coating composition, the resulting coating must adhere strongly to the base, must dry or cure bubble free, must produce in a single application a heavy coating that is highly resistant to both abrasion and asphalt staining.
Coating compositions have been developed which, when applied to a substrate and cured, impart a highly abrasion resistant surface to the substrate. Coating compositions of this type have been widely used to impart abrasion resistance to plastic lenses such as eyeglass lenses, to plastic panels and films, to wood surfaces such as furniture, and many other applications where an abrasion resistant or scratch resistant surface finish is of importance.
Abrasion resistant coatings of this type are typically based upon acrylate monomers which are cured or cross-linked after application of the coating, typically by radiation curing. Radiation curable coatings offer the advantage of being rapidly cured and polymerized without requiring curing ovens and they can be applied and processed without having to remove solvents and deal with solvent vapors in the workplace environment.
It is known that radiation cured acrylate polymers can produce very hard (glass hard) protective coatings which exhibit superior abrasion and chemical resistance properties. Although the coatings are quite hard and resistant to abrasion and scratching, they are brittle and have a tendency to crack and peel from the substrate, especially when applied to relatively flexible substrates or when subjected to impact.
Prior abrasion resistant coatings have sought to deal with the brittleness and cracking problem by using a softening comonomer (a monomer with a low second order transition temperature) to impart some degree of flexibility to the coating. However, in achieving increased flexibility and reduced brittleness, the abrasion resistance of the coating is sacrificed. Thus, for example, U.S. Pat. No. 4,319,811 describes an abrasion resistant radiation curable coating based upon tri- and tetra acrylate monomers, such as pentaerythritol triacrylate with a comonomer such as vinyl pyrrolidone or vinyl caprolactam. U.S. Pat. No. 4,308,119 teaches an abrasion resistant radiation curable coating composition comprised of a pentaerythritol tetra acrylate with a cellulose ester such as cellulose acetate butyrate. U.S. Pat. No. 4,557,980 discloses a radiation curable coating composition based upon a mixture of a triacrylate or tetra acrylate, such as pentaerythritol tetra acrylate, with acrylic acid.
The resistance of a coating to scratching abrasion is typically measured by the rotary steel wool test, which involves subjecting the coating to five revolutions of a pad of 0000 grade steel wool at a defined pressure, usually 12 or 24 psi. The scratching abrasion resistance is rated by measuring the increase in haze from the abrasion. Test methods such as ASTM D-1044 have been developed for optically measuring the resistance of transparent plastic materials to abrasion. Other standard tests for abrasion resistance are the Taber abrasion test described in ASTM D-1004-56.
In many applications, the protective finish needs not only to be "hard" and thus resistant to scratching, but also must have excellent toughness and resistance to impact. The toughness or impact abrasion resistance of a coating is commonly measured by the "falling sand" test (ASTM D968-51). A coating which has good scratch abrasion resistance may not necessarily have good impact abrasion resistance. With the falling sand test, sand is poured onto a coating from a predetermined height, while the thickness of the coating is observed. The results are expressed in terms of the number of liters of sand required to abrade away one tenth of a mil of the coating thickness. The radiation cured abrasion resistance coatings noted in the aforementioned prior patents have a relatively poor resistance to impact abrasion which renders these types of coatings unacceptable for applications requiring both good resistance to scratching abrasion an good resistance to impact abrasion.
The prior art is silent regarding coatings which incorporate a filler having a particle size in the range of 1-20 microns capable of imparting abrasion resistance to a coated substrate.